RFR: 8270447: [IR Framework] Add missing compilation level restriction when using FlipC1C2 stress option

[Christian Hagedorn]

A stress test with the IR framework option `FlipC1C2` for the newly converted Valhalla tests ([JDK-8263024](https://bugs.openjdk.java.net/browse/JDK-8263024)) resulted in invalid compilation requests (an exception is thrown) for unavailable compiler levels. A compilation level restriction after the flip from C1 -> C2 and vice versa was missing and also an exclusion when asserting on specific compilation levels with `DFlipC1C2`. I also included some small style fixes.

Thanks,
Christian

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Commit messages:
 - [IR Framework] Add missing compilation level restriction when using FlipC1C2 stress option

Changes: https://git.openjdk.java.net/jdk/pull/4866/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4866&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270447
  Stats: 17 lines in 1 file changed: 2 ins; 0 del; 15 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4866.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4866/head:pull/4866

PR: https://git.openjdk.java.net/jdk/pull/4866